Photochromism is a property of certain classes of molecules in which visible or ultraviolet light leads to the reversible isomerization of the molecule to another form having a different UV-visible absorption spectrum. Compounds of this type have found widespread use in sunlight-responsive optical articles, such as ophthalmic lenses, goggles, face shields, windows, aircraft transparencies, and display screens. Photochromic molecules can be loosely divided into two types: those that return to their initial state at a rate dependent on the ambient temperature (“thermally reversible” photochromic materials), and those which are thermally stable, requiring light absorption of a different wavelength to return to their initial state.
It is apparent that for the type of photochromic molecules that thermally return to their initial state, the equilibrium between isomeric forms is a function of both light intensity incident and the temperature of the matrix in which they are contained. This is the case for photochromic molecules typically incorporated into optical articles such as those previously mentioned. That is, for such thermally reversible photochromic molecules, their degree of activation (degree of coloring) displays a temperature dependent response. For reasons both cosmetic and practical it is desirable to mitigate this temperature dependence to some extent.